The present invention relates to an improved anti-siphon and back flow prevention valve for use in plumbing systems and more specifically to such a valve having an air valve and check valve where both valves operate independently of each other.
Anti-siphon and back flow prevention valves have been in use for some time and are generally used in plumbing systems. Such valves are often required by local health and sanitary codes. The valves prevent the contamination of the liquid supply upstream from the valve due to liquid back-flow and contamination of the supply line downstream from the valve due to siphoning.
Such valves commonly incorporate an air valve to permit the entry of air into line under pressure conditions that would otherwise permit a liquid to siphon upstream to the valve. They also incorporate a check valve to prevent liquid from passing upstream through the valve to the supply.
Such valves frequently use diaphragms, springs and cams to detect water pressure upstream from the valve and to close a normally open air valve before liquid reaches the valve, in order to prevent the liquid from leaking through the air valve during normal flow conditions. In such valves the possibility of leakage under low liquid pressure conditions or when the flow is started is always present. Moreover, these valves are often large and expensive and tend to reduce the liquid pressure across the valve. This renders these valves particularly unsuitable for plumbing installations in areas of limited space and possible low liquid pressure conditions such as in mobile homes and other similar recreational vehicles. The possibility of leakage from the air ports under low liquid pressure conditions likewise limits the utility and consumer acceptance of such valves in these applications.
Most existing valves use a single diaphragm to provide both the air valve and check valve functions and are therefore subject to the above limitations, notably leakage during low pressure conditions and start-up and pressure reduction through the valve. U.S. Pat. No. 2,174,742 discloses a valve which incorporates separate diaphragms for the air valve and check valve. In this valve, however, both diaphragms are normally open during no-flow conditions. The check valve closes only upon the start of backflow, thereby permitting an initial contamination of the line upstream. The air valve requires a finite amount of liquid pressure acting on it before it closes off its air ports completely. Therefore, leakage of liquid through the air valve is possible under low liquid pressure conditions or when the liquid flow has initially started.